I have a MacBook Pro running Mavericks.  Sibelius 7.5, and I have VDL soundset 7.0a in my playback devices.

When I have a Rack Combo A part, and then put in an instrument change to Rack Combo B, the rack combo B plays back general midi maps instead of the Rack Combo B sounds/maps. When I instrument change to BD/TamTam combo, it plays back other bass and tam tam sounds that are not of the VDL bass/tamtam soundest. Every button is either a bass hit or tam tam hit. The typical variety of bass hits and rolls, along with tamtam rolls are not there and do not play back. Every key is either just a bass hit or tam tam hit.  Anyone else have this issue?

The first thing I'd check is to add another instance of Kontakt Player to your Sibelius playback configuration. It's possible that Sibelius's auto-load thing may be trying to use a different patch in place of your desired one because it's run out of available channels (there are 16 channels (instruments) available per instance of KP).

If that doesn't do it, I'd suggest checking with the folks at [url=http://www.thewritescore.com]The Write Score[/url] since this sounds like it's an issue related to how you're implementing your template.